The full history is a long and complex story, but in short, Taylor and Katy fell out in a big way after Katy allegedly hired Taylor's dancers in the middle of Taylor's tour. In an interview with Rolling Stone, Taylor said the experience made her realise they were "straight-up enemies", and she reportedly wrote the track "Bad Blood" about it.

The tweet referred to Taylor's song "Bad Blood", which is rumoured to be about her fallout with Katy. The track was released as a single, and Taylor enlisted most of her female celebrity friends to star in the video.

After Taylor apologised to Nicki and Nicki forgave her, everyone assumed everything would go back to normal.

But then during one of Taylor's performances on her 1989 world tour, a shark was brought out for her rendition of "Bad Blood”.

Many people took this to be a reference to Katy Perry's now infamous performance at the Super Bowl, where a backing dancer dressed as a shark forgot the choreography and the "Left Shark" meme was born.

And now earlier this week, Katy Perry was caught on Instagram following an account called "sluttytaylorswift".

The account belongs to a user called Dinah who appears to have no link to Taylor and posts photos like this. ¯\_(ツ)_/¯

And despite many people noticing, it still remains at the top of Katy's "Following" list, which only consists of 202 users. The fact she hasn't unfollowed the account seems to suggest that she's not particularly bothered who sees it.